<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
    <h1>0</h1>
    <h2>0</h2>

    <script>
        let count = 0;
        //开启定时器,每隔一秒调用一次f方法
        setInterval(f,1000);
        function f() {
            count++;
            let h = document.querySelector("h1");
            h.innerText = count;
        }

        let num = 0;
        //匿名方法开启定时器
        let timer = setInterval(function () {
            num++;
            let h = document.querySelector("h2");
            h.innerText = num;
            if (num == 50) {
                clearInterval(timer);
            }
        },100);

        //开启只执行一次的定时器
        setTimeout(function (){
            alert("时间到!");
        },3000);
    </script>
</body>
</html>